This invention relates to a method for coloring stainless steel, whereby stainless steel can be colored to a desired color tone with a satisfactory reproducibility even when it has a relatively uneven surface.
Recently, the techniques of coloring stainless steel have progressed rapidly, and in accordance with the increase in the production of colored stainless steel, it is now used in various fields. Among the conventional techniques, the method of coloring stainless steel by controlling potential difference between a reference electrode and the surface of stainless steel to be colored (which was developed by Inco Europe Limited in Britain) is particularly good in respect of reproducibility of color tone and hardwearing properties. This method is fully described in Japanese Patent Publication (Tokko-sho) No. 52-25817 entitled "Method of Treating Chromium Alloys", which particularly fully discloses a method for controlling the reproducibility of color of stainless steel. The method disclosed therein comprises monitoring the potential difference between the surface of the stainless steel being colored and a reference electrode (such as a saturated calomel electrode or a platinum electrode and the like) in an aqueous solution of chromic and sulphuric acids; determining the inflexion potential at the inflexion point A at which the desired color begins to form on the metal surface and the finish potential B at which the formation of the desired color is complete on the basis of the potential-time curve showing the variation with time of the potential of the stainless steel, measured against the reference electrode, as given in FIG. 1; and removing the colored stainless steel from the coloring solution when the potential is varied from the inflexion point A to the potential B in accordance with the predetermined amount. That is, according to this method, the desired color tone can be obtained when the potential is varied from the inflexion point A to the potential B, and this variation of the potential difference appears constantly between the inflexion point A and the potential B, thus a constant color tone being produced by controlling the potential difference (B-A) from the inflexion point A.
However, it has been found that the appearance of the inflexion point A on the potential-time curve of FIG. 1 in accordance with this method varies depending on the state of the surface finish of stainless steel to be colored. Sometimes, this appearance is very indistinct, and in an extreme case the inflexion point A does not appear. When a stainless steel to be colored is subjected to bright annealing finish, very fine mirror polishing finish or 2B finish of skin pass finish by rolls having gloss after cold rolling, the surface of the stainless steel becomes relatively even, and the inflexion point A appears distinctly on the potential-time curve of FIG. 1. However, when the surface of a stainless steel to be colored is uneven, for example, when the stainless steel is subjected to a coarse abrading finish or HL finish by using a coarse abrasive material to leave a continuous abrading trace, the inflexion point A does not appear on the potential-time curve as shown in FIG. 1 but the curve as shown in FIG. 2 appears. That is, FIG. 2 shows an example of the potential-time curve showing the variation with time of the potential difference between the surface of an HL-finished SUS 304 stainless steel and a reference electrode, and in this case, the inflexion point A as shown in FIG. 1 does not appear.
As mentioned above, if the surface condition of a stainless steel is unacceptably bad, it is impossible to determine the inflexion point A from the potential-time curve, and it is therefore very difficult to control the color tone on the basis of the inflexion point A on the potential-time curve showing the variation with time of the potential difference between the surface of a stainless steel and a reference electrode. Thus, according to this method of the prior art, it is impossible to obtain a desired color tone with a satisfactory reproducibility when the surface condition of a stainless steel is unacceptably bad. Therefore, when a stainless steel to be colored has a relatively uneven surface such as a surface of HL finish, the stainless steel must be subjected to an electrolytic treatment, for example, by dipping the stainless steel in an aqueous solution of phosphoric acid before coloring in order to make the surface of the steel substantially even by forming a passive film on the surface, so that the inflexion point A as shown on the potential-time curve of FIG. 1, which is the key to controlling color tone, can appear distinctly when the pre-treated stainless steel is dipped in a coloring liquor comprising a mixed aqueous solution of chromic and sulfuric acids. Alternatively, the coloring is controlled simply by the dipping time in the coloring liquor at the expense of reproducibility of the desired color.
We have fully studied the potential-time curve showing the variation with time of the potential difference between the surface of a stainless steel and a reference electrode in this coloring liquor, and have discovered that the inflexion point in question can be distinctly located even from the potential-time curve having no distinct inflexion point as shown in FIG. 2 by measuring a variation amount of potential per unit time, i.e. by differentiating the variation with time of the potential by time to prepare the differentiation curve thereof. Thus, we have found that a desired color tone can be obtained with a satisfactory reproducibility even in the case of a stainless steel having a relatively uneven and ununiform surface by controlling the coloring in accordance with this differentiation curve.